FreeBSD i386_set_ldt Integer Signedness Vulnerability iDefense Security Advisory 09.23.06 http://www.idefense.com/intelligence/vulnerabilities/ Sep 23, 2006 I. BACKGROUND FreeBSD is a modern operating system for x86, amd64, Alpha, IA-64, PC-98 and SPARC architectures. It's based on the UNIX operating system, BSD, which was created at the University of California, Berkeley. More information can be obtained from the FreeBSD Project web site at http://www.FreeBSD.org/ II. DESCRIPTION Local exploitation of an integer signedness vulnerability in version 5.4 of FreeBSD allows attackers to create a denial of service condition. The i386_set_ldt() call is provided to allow user-land programs to dynamically manage the per-process local descriptor table. Due to the use of signed integers and a lack of proper input validation, a situation can occur in the kernel where bzero is called with a very large size parameter. The affected code follows. 415 int error = 0, i; 416 int largest_ld; .. 449 largest_ld = uap->start + uap->num; 450 if (largest_ld > pldt->ldt_len) 451 largest_ld = pldt->ldt_len; 452 i = largest_ld - uap->start; 453 bzero(&((union descriptor *)(pldt->ldt_base))[uap->start], 454 sizeof(union descriptor) * i); As seen on lines 415 and 416, the 'i' and 'largest_ld' variables are both signed integers. On line 449, uap->start and uap->num are added together. It should be noted that both of these variables are user controlled and are not checked properly. On line 452, 'i' can be set to a negative value which causes the bzero on line 453 to be called with a very large length parameter. Invalid memory access will result in a kernel panic. III. ANALYSIS Exploitation of this vulnerability would result in a denial of service condition on the affected host. IV. DETECTION iDefense has confirmed the existence of this problem in FreeBSD 5.5. It is suspected that versions as early as 5.2 are also affected. V. WORKAROUND iDefense is not aware of any workarounds for this issue. VI. VENDOR RESPONSE "The policy of the FreeBSD Security Team is to not issue security advisories for local denial of service attacks; since we have not been able to demonstrate that this bug can result in anything more severe than a denial of service, we will not be issuing a security advisory relating to this problem. It is possible that an Errata Notice will be issued concerning this problem." VII. CVE INFORMATION The Common Vulnerabilities and Exposures (CVE) project has assigned the name CVE-2006-4178 to this issue. This is a candidate for inclusion in the CVE list (http://cve.mitre.org), which standardizes names for security problems. VIII. DISCLOSURE TIMELINE 08/16/2006 Initial vendor notification 08/16/2006 Initial vendor response 09/23/2006 Public disclosure IX. CREDIT The discoverer of this vulnerability wishes to remain anonymous. Get paid for vulnerability research http://www.idefense.com/methodology/vulnerability/vcp.php Free tools, research and upcoming events http://labs.idefense.com/ X. LEGAL NOTICES Copyright © 2006 iDefense, Inc. Permission is granted for the redistribution of this alert electronically. It may not be edited in any way without the express written consent of iDefense. If you wish to reprint the whole or any part of this alert in any other medium other than electronically, please email customerservice@private for permission. Disclaimer: The information in the advisory is believed to be accurate at the time of publishing based on currently available information. Use of the information constitutes acceptance for use in an AS IS condition. There are no warranties with regard to this information. Neither the author nor the publisher accepts any liability for any direct, indirect, or consequential loss or damage arising from use of, or reliance on, this information.
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